1. Field of the Invention
This invention relates to a liquid drop rate controller for controlling the flow of liquid from an infusion fluid reservoir (such as a drip-bottle or bag) to an intravenous or other infusion site of a patient in medical care.
2. Description of the Prior Art
Several types of drop rate controller are known whereby the rate of liquid infused into a patient may be variably controlled and maintained at a preselected rate. A common type of controller involves variation in the rate of pumping of the liquid along a flexible tube between the liquid reservoir and the patient. Typically the pumping action is provided by a peristatic pump, the motor of which is electronically controlled in relation to a desired, preset liquid supply rate. The latter rate is commonly expressed as a drip-rate, with a specific number of drips per unit time being set as the desired liquid supply rate. A control of the drip-rate by means of a pumping action has proved popular since pumps are generally easy to control electronically whereby to supply desired liquid supply rates, but currently a pumping action of infusion liquid into the human body is meeting with resistance in some medical circles, who advocate that a gravity-feed infusion system is safer and more reliable. Unfortunately electronic control of gravity feed mechanisms which do not introduce an element of pumping to the liquid are not easy to design.
Despite difficulties, some electronically-operating gravity-feed infusion controllers do exist. A typical one is the Series 200 Infusion Controller produced by Ivac Corporation, U.S.A. This controller senses the drip-rate of drops as they fall under gravity from a drip reservoir (e.g. a bag) into a chamber holding a head of the liquid. The liquid subsequently passes through a flow restrictor to the patient. Should the hydrostatic pressure of the liquid being infused alter (for example as a consequence of a change in the intravenous back-pressure from the patient's blood system) and the drip-rate change in consequence, the change in drip-rate is sensed by the controller and the hydraulic impedance of the flow restrictor altered to restore the drip-rate to its preset level. The hydraulic impedance of the flow restrictor is essentially created by mechanically compressing a flexible tube through which the liquid passes, the compression being converted from electrical signals to mechanical compression through a solenoid. The solenoid is continuously pulsed with power and thus the compression supplied to the tube is a continuous mechanical pulsing of the tube. The hydraulic impedance is changed by altering the electrical pulse signals supplied to the solenoid and thus altering the frequency/pulse width of the mechanical pulses compressing the tube.
One disadvantage of the present gravity-feed infusion controllers is that the dynamic range of preset drip-rates over which they can operate is not wide and, especially, they do not operate efficiently or at all at very low drip-rates.